1. Fixation: CO2 reacts with 5C molecule (RuBP) catalyzed by Rubisco enzyme new 6C split into 2 acids (3C each) 2. ATP and NADPH turn 3C acids into sugars reduction of acids 3. Some 3C sugars recycled to re-make RuBP (5C) one 3C sugar leaves to make glucose recycling needs ATP
Most abundant enzyme in the world Most CO2 converted into biomass is fixed by Rubisco Large molecule : 16 polypeptides : 8 active sites
Only 3-10 reactions per second limits rate of photosynthesis 20% error rate gets worse at high temperatures Also reacts with O2 (cheater!) O2 reaction called Photorespiration uses ATP and NADPH to make CO2
Evolution doesn’t always create the best solution Photorespiration is completely wasteful 2x energy to produce the same amount of sugar than if Rubisco only reacted with CO2 Rubisco has evolved high affinity for CO2 (80x > O2) compare to the composition of the atmosphere…
Why is there so much Rubsico? Why is it so big and slow? Was Rubisco always a cheater? Why did the atmosphere change?
Evolution of enzymes difficult favors stability selection against modifying mutations RubisCO → RubisC??? requires a decrease in fitness plants are stuck with Rubisco Natural selection increased affinity for CO2 made Rubisco slow
Changed existing parts and pieces work around the photorespiration issue In response to environmental change lower [CO2] & higher [O2] high temperatures water limitations (stomata) Evolution of new photosynthesis pathways Allowed plants to exploit drier habitats
Calvin cycle moved to Bundle Sheath Cells surrounding leaf veins chloroplasts now present BSC cells are less permeable to gases what does this mean for photorespiration?
C4 mechanism separates Calvin cycle from high O2 New fixation pathway in mesophyll cells New enzyme, phosphoenolpyruvate carboxlylase (PEPC) fixes CO2 to PEP (no affinity for O2) new 4C malate pumped to bundle sheath cells malate broken down to release CO2 3C molecule transported back to mesophyll Hyper efficient with use of CO2 How does this impact stomata behavior?
Regeneration of PEP expensive (ATP) in addition to ATP needed for Calvin cycle Where is needed ATP generated? Where should C4 plants exist?
In hot conditions, the benefits of reduced photorespiration likely exceed the ATP cost of moving CO2 from the mesophyll cell to the bundle-sheath cell
Stomata open at night only 4C compounds produced in large quantities stored when Rubisco is inactive C4 vs CAM substitute BSC for “night shift”
CAM evolved independently several times ~16,000 species Exist in arid environments or habitats cacti epiphytes Are CAM plants fast or slow growing?
C4 plants are economically important corn, sugarcane, sorghum & switchgrass So are C3 crops beans, rice, wheat, potatoes (temperate crops) Global water use is set to triple by 2050 70% to the agriculture sector 50% transpired through stomatal pores